More than a soft touch and a pretty face

No longer just for aesthetics, multibarrel, multimaterial coinjection molding is gaining ground in structural applications.

Steve Kolste Bemis Mfg. Sheboygan Falls, Wis.

Chris Scarazzo Solvay Advanced Polymers LLC Alpharetta, Ga.

It’s a processing technique that’s been around since the early ’70s. But coinjection molding and its offspring technologies have never been more in vogue. Their prominence comes from parallel advance in multibarrel, coinjection molding and the seemingly weekly breakthroughs in materials.

These molding techniques address a problem with high-performance plastics. They are often too expensive to serve as the sole material. But multibarrel coinjection molding lets molders economically put these advanced materials on the outside of the part while using less costly or recycled materials inside.

The process uses one machine, one tool, and a single cycle to mold two materials, separate and distinct, or combined into one product. Coinjection basically laminates the materials to create a structure that is often stronger and more stable than would be possible with a single material.

Lower costs and expanded material options have been the key drivers behind the spread of coinjection molding. Designers traditionally used the process for aesthetic applications. There is less chance a consumer would get hurt if, for example, the colorful soft touch grip delaminated from a toothbrush. Likewise, tactile accents used on handles and grips provide ergonomic benefits such as reducing vibrations and fatigue, but often play little or no structural role. Therefore, in a profession that prides itself on caution and precision, critical components would see no untried material.

But designers jumped at the opportunity to evaluate new plastics that could give aesthetics over those possible from metal or composites. This was especially true where molded-in-color could replace commercial painting.

However, designers are widening plastics’ structural role as metal prices continue to rise and new plastics deliver better mechanical properties and heat and chemical resistance.

For example, Ixef polyarylamide from Solvay Advanced Polymers L.L.C., Alpharetta, Ga., is highly prized for aesthetic looks and excellent stiffness. It has found its way into a wide array of consumer products including cell phones and PDAs. However, there’s a premium price for those good looks and performance. But coinjection molding with, for example, PBT regrind in the core of a part can address that cost issue.

Designers are now embracing coinjection molded or multishot parts for more than aesthetics. Consider a part with a cross-sectional area of 1 sq in. If this part were made from a material that had a tensile strength of 30 kpsi, it would theoretically have a load bearing capability of 30,000 lb.

However, in practical terms it is nearly impossible to mold a part with such a large cross-sectional area without defects. In the molding process, the outer perimeter of the part is the first to freeze or cool. The inside is still in its melted state though the perimeter has been established. As the part cools, the inside wants to shrink. Because the outside of the part has a firm dimension, the inside sees stresses that ultimately causes defects such as sink marks, internal stresses, cracks, or voids.

Sink marks are easy to spot on a part surface, but voids and cracks are internalized and can go unnoticed. Internal defects, however, significantly detract from the overall mechanical properties of the part. These defects act as stress risers and are the points of failure initiation. The reality of such a part is that it will typically only have around half the design load-bearing capability.

Traditionally, designers have addressed this problem by coring out or removing material from the cross section of the part using a gas-assist molding process. This results in a hollow part with uniform walls that can be molded without defects. It is here that the design engineer sometimes clashes with the product designer. That’s because the product designer may be more concerned with the aesthetics of a part that “feels” hollowed out, or lacks the bulk or mass normally associated with a critical, structural part.

In contrast, coinjection molding lets designers mold an aesthetic, high-strength “skin” on a part while the core is molded in a lowerperforming, less-costly material that can also be foamed. Foaming the core produces a uniform cell structure which can have reasonably good mechanical properties, yet be free of the stresses and defects of a fully densified part.

Consider that 1-sq-in. cross section again. Assume a 50-50 breakdown of skin and core material. If one has a core material having a tensile strength of 15 kpsi with an exterior material that has 30 kpsi, the practical strength will be 50% greater than in a part made from a single material.

Further, the part is lighter and its materials cost less. Regrind from the molding process can also be incorporated into the core material, further improving efficiency without sacrificing appearance.

In the past decade, demands to reduce weight and cost, have pushed designers to consider polymers for complex and critical components such as gears, bushings, bearings and even support structures such as seating. New advances in processing, including gas assisted, multimaterial molding and new grades of high-performing plastics have made this possible.

New plastics stand up to extreme temperatures and demanding environments that were once only the domain of metals and composites. In addition, emerging tiers of “ultra” polymers can deliver compressive performance once possible only in a metal like titanium.

Just think about a typical riding lawn mower. A model made in 1990 was bulky, boxy, heavy, and by now, probably rusty. By contrast, today’s models are sleek, user friendly, easier to maneuver, and more fuel efficient. No design engineer worth his or her salt would have ever considered grill, lighting, or enginepanel components made of plastic 15 years ago. That just isn’t the case today.

Such trends come from the ever expanding performance profile of plastics and the evolution of cost-effective processing techniques such as multibarrel molding.